This invention relates to steering systems for automotive vehicles and more particularly to a rear-wheel steering device for an automotive vehicle (hereinafter referred to as vehicle or car) having a four-wheel steering system.
Steering by means of the steering wheel to steer the front wheels and also to steer the rear wheels is known. In a vehicle of this type, the ratio between the rear-wheel steering angle .theta..sub.r and the front-wheel steering angle .theta..sub.f, that is, the ratio k(=.theta..sub.r /.theta..sub.f) is variable in accordance with the driving speed.
At low speeds, the rear wheels are caused to turn in the reverse direction of that of the front wheels (so-called "reverse-phase steering") for the purpose of improving the vehicle turning characteristic. In the medium and high speed range, the rear wheels are caused to turn in the same direction as that of the front wheels (so-called "coincident-phase steering"), and at the same time, as the vehicle speed increases, control is so carried out that the rear/front wheel steering angle ratio k mentioned above gradually approaches 1 (one thereby to improve the stability of the vehicle with respect to steering maneuvers during high-speed driving. Four-wheel steering systems having these features have been developed in the prior art as disclosed in Japanese Patent Laid-Open Publication No. 11173/1982.
In spite of the progress made in the prior art, a number of unsolved problems still remain. For example, in a four-wheel steering system of this type there is a need for the provision of fail-safe devices such as a sensor for detecting the front wheel steering angle and transmitting a corresponding signal to a control unit and for the provision in this control unit of diagnostic means which can detect any abnormality or malfunctioning such as a short-circuit or a conductor breakage in the various circuits connected to this control unit. Such diagnostic means can operate in instant response to such detection to stop the rear-wheel steering control and place the steering system in the normal condition having no rear-wheel steering means, that is, in the condition wherein only the front wheels are steered by the manipulation of the steering wheel.
In one example of a fail-safe device provided in a known rear wheel steering system, in order to improve the capability of detecting abnormalities, a plurality of sensors, for example, for detecting the front wheel steering angle and steering speed of the front wheel are provided, and the outputs of these sensors are compared. When only one front wheel steering angle sensor is used, whether or not its output is within the normal operational range specific to this sensor is verified in another method. These are ordinarily practiced methods. In the case of the former method, high cost becomes a problem. In the case of the latter method, there is a problem in that detection of differences in the polarity of the front wheel steering angle signal transmitted from the front-wheel steering angle sensor is difficult. That is, even when the steering wheel is manipulated for a right-turn, a front-wheel steering angle signal indicates a left-turn steering condition. If this output is within the aforementioned normal operational range, it cannot be detected as being abnormal.